Got "SyntaxError: Unexpected token e in JSON at position 0" message when use supertest to get token

Question

I use supertest to test web API, and I created a function to get the token,

module.exports.getToken = () => {
    let query =
        {
            "email": config.dev.adminUsername,
            "password": config.dev.adminPassword,
            "applicationId": config.dev.applicationId
        }
    request(config.dev.loginUrl)
        .post('/login')
        .send(query)
        .expect(200)
        .then(res => {
             console.log(res);
         })
         .catch(err => {
            console.log(err);
         });
}

but I got error looks like,

{ SyntaxError: Unexpected token e in JSON at position 0
    at JSON.parse (<anonymous>)
    at IncomingMessage.res.on (C:\WorkSpace\Postman\saasify-integration-tests\supertest_testScripts\node_modules\superagent\lib\node\parsers\json.js:11:35)
    at emitNone (events.js:111:20)
    at IncomingMessage.emit (events.js:208:7)
    at endReadableNT (_stream_readable.js:1064:12)
    at _combinedTickCallback (internal/process/next_tick.js:139:11)
    at process._tickCallback (internal/process/next_tick.js:181:9)
  rawResponse: 'eyJraWQiOiJ4eGh5cXV3.................FRfeCpAm3sYxY7fV-WDK1w',
  statusCode: 200,
  response: undefined }

But same request and url in postman I can get the token string without error. what I did wrong?